According to an examination by the inventor of the present invention, the followings are known as the technologies for a memory using a phase change material.
Phase change memories have been under development in order to produce high-speed and large-scale integrated non-volatile memories. A memory (phase change memory) employing resistive elements made of a phase change material is a non-volatile memory, in which a temperature change such as that shown in FIG. 31 is caused by use of electrical pulses so as to cause a phase transition of a phase change material between an amorphous state and a crystalline state, and by doing so, the difference in resistance values between the amorphous state (reset) and the crystalline state (set) of the phase change material is recorded as information as shown in FIG. 32. Note that, for the high resistance value of the amorphous state and the low resistance value of the crystalline state of the phase change material, a completely amorphous state and a completely crystalline state are not required. As a storage element, it is important to have sufficient difference in resistance between a high resistance state and a low resistance state, and the resistance values can be arbitrary intermediate values between a completely amorphous high-resistance state and a completely crystalline low-resistance state.
As described above, the phase change element changes the phase state thereof by electrical pulses. In order to perform a reset, a large current has to flow for a short period so as to rapidly cool the element. On the other hand, in order to perform a set, a current smaller than that in the reset has to flow for a comparatively long time so as to slowly cool the element. Meanwhile, in a read-out operation, the speed of a voltage drop in a bit line is sensed by use of a read voltage which is sufficiently lower than the write voltages, thereby reading out a ‘0’ state or a ‘1’ state of the phase change memory.
As a read-out method of a phase change memory, for example, a technology that enables easy setting of a sense amplifier reference level by amplifying input signals to the sense amplifier by use of a charge-transfer type preamplifier is disclosed in “2004 IEEE International Solid-State Circuits Conference, Digest of Technical Papers”, pp. 40 to 41 (Non-Patent Document 1).